This invention relates to digital circuit testing apparatus and particularly to a system for testing circuit boards containing microprocessor units.
In the specialized field of digital circuit diagnostic analysis, there has developed a need for a capability to rapidly and automatically test complex circuit boards under operating conditions. Automatic testers are known for analyzing the operation of discrete components or integrated circuit components which may be mounted to circuit boards as part of a more complex system.
Heretofore, it has not been possible to test microprocessor based boards under normal operating conditions or in a simulated operating environment because of the unique capabilities and demands of microprocessor based systems. Microprocessors are often considered in the art to be "intelligent", that is, a microprocessor unit in its operational environment has the capability of responding to digital data from and providing a digitally processed output to peripheral circuit units such that real time, quasi-intelligent interaction between the system and the external environment is possible. Furthermore, the relatively small cost and compact size of a microprocessor makes dedicated application of computerized systems a practical possibility. A general purpose digital computer is unsuited in many such dedicated applications. A microprocessor based system is generally not independent of its environment. Therefore, there is a need to test microprocessor-based circuit boards in an interactive environment simulating operating conditions.
General purpose digital computers have been suggested as a means for testing microprocessor based systems. However, a general purpose digital computer based system, which is preprogrammed to exercise a microprocessor based system in a diagnostic environment is typically too slow in operation to effectively interact with the microprocessor based system. Furthermore, the general purpose digital computers are generally not capable of operating under the timing control of external circuitry, such as a microprocessor based system. Thus, general purpose digital computer based systems are unsuited for thoroughly testing microprocessor based systems in real time.